Known high-altitude long endurance (HALE) aircraft which carry fuel, for example the Northrop Grumman's Global Hawk, are limited to one day of operation. NASA and DARPA have been pursuing the development of HALE aircraft which carry no fuel, and which are also know as “atmospheric satellites”, including for example Boeing's SolarEagle. These craft are designed to collect solar power for propulsion and operations. Consequently, these aircraft must store large amounts of energy during the day to run propulsion during the night, requiring very large flexible wing covered in solar cells and heavy batteries. In the case of AeroVironment's Helios, a structural failure has resulted.
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